Gene “boosts” hope for plant-based jet fuel, plentiful food

A team of scientists has identified a gene in poplar trees that enhances photosynthesis and can boost tree height by as much as 200 percent.

Their discovery is a game-changer for national efforts to grow poplar trees that can be used for jet fuel as an alternative to petroleum-based fuel, one of the scientists said. UC Davis is part of those efforts, with nearly a thousand poplar trees being grown for genetic analysis by members of the lab of Gail Taylor, a distinguished professor emerita in the department and now dean of the Faculty of Life Sciences at University College London.

Discovery of the gene also raises hopes of gaining higher yields from important food crops.

The gene, which the scientists dubbed Booster, occurs naturally in poplar trees. They found it enhances photosynthesis — plants’ process for turning sunlight, water and carbon dioxide into food — and, therefore, boosts plant growth. The gene contains DNA from two associated organisms found within the tree, and from a protein called Rubisco that is essential to photosynthesis.

The scientists were studying a type of poplar — the black cottonwood tree, or Populus trichocarpa, which thrives from Mexico’s Baja California into northern Canada. Its range includes areas where land is poor and water is scarce, making it exciting for research as climate warms and rain/snow patterns change. This tree is a leading candidate for making fuel and other products by breaking down its wood and distilling the mash.

In experiments with poplars with the Booster gene enhanced, trees grew about 30 percent taller in the field and as much as 200 percent taller in the greenhouse.

Booster also increased the size of another plant species, thale cress, or Arabidopsis. Thale cress is a small, leafy, green weed that grows all over Europe, Asia and Africa. It's also model species used widely in research, so discovering the gene in that plant, too, points to the possibility of gaining higher yields from food crops that might also have Booster.

The research comes from two Department of Energy Bioenergy Research Centers — the Center for Bioenergy Innovation at Oak Ridge National Laboratory in Tennessee, and the Center for Advanced Bioenergy and Bioproducts Innovation at the University of Illinois, Urbana-Champaign. Their discovery was described this week in the journal Developmental Cell.

Booster: Three genes that evolved into one

In Greek mythology, a chimera was a female fire-breathing monster with a lion's head, a goat's body and a serpent's tail.

Likewise, Booster is a chimeric gene: It contains DNA sequences from three genes that, originally, were separate, but which combined, largely unchanged, into a single gene over eons of time. More broadly, chimeric genes have unique origins and are thought to enable evolutionary changes that help plants adapt to new environments.

In the case of Booster, the ORNL team determined that it contains one segment from a bacteria found in the poplar tree’s root system. Another segment comes from an ant that farms a fungus known to infect poplars. The third segment is from the large subunit of Rubisco, an abundant protein found in plant chloroplasts.

Chloroplasts are the principal cell structures that house the photosynthetic machinery converting light energy into glucose — the building block for cellulose, starch and other large molecules that plants use to create stems, leaves, seeds and fruit. And so, they're also related to both food and fuel production.

The Rubisco protein functions as the plant’s “carbon-grabber,” capturing carbon dioxide from the air. For years, scientists have been working on ways to boost the amount of Rubisco in plants for greater crop yield and absorption of atmospheric CO2.

When the researchers created poplar trees with greater expression of the Booster gene, their Rubisco content and subsequent photosynthetic activity soared. As a result, trees  grew as much as 200 percent taller in greenhouses.

The trees also had up to 62 percent more Rubisco, and their leaves showed an increase of about 25 percent in net CO2 uptake.

Among boosted poplars grown in fields, trees grew up to 37 percent taller, with as much as 88 percent more stem volume, increasing biomass per plant.

This means the Booster gene can increase plant yield without using more land, water or fertilizer. That would support a robust economy revolving around plant-based fuels.

Many benefits from one enhancement

Poplar and Arabidopsis are C3 plants, a category that includes key food crops such as soybeans, rice, wheat and oats. If Booster works the same way in food crops, higher yields could reduce food scarcity around the world.

To explore that idea, scientists also inserted Booster into Arabidopsis. The result was a similar increase in biomass and a 50-percent increase in seed production. This indicates Booster could, potentially, trigger higher yields in other plants.

“Growing high-yielding, perennial bioenergy crops on marginal lands unsuitable for conventional agriculture can help us meet rising demand for liquid biofuels for hard-to-electrify sectors like aviation,” said Gerald A. Tuskan, CBI director and a corporate fellow at ORNL who coauthored the paper. “Fast-growing, resilient feedstock plants can stimulate the bioeconomy, create rural jobs and support forecasted demand for energy.”

“This discovery could be a game-changer in terms of a big stimulation of photosynthesis and plant productivity,” added Stephen Long, a leading authority on plant photosynthesis and professor at the University of Illinois Urbana-Champaign. He is a coauthor on the paper in his role with the Illinois-led CABBI. “While we need to test more widely to be sure we can reproduce the results on a large scale, the fact that it worked in a completely unrelated plant indicates that it could work over a wider range of plants.”

Next steps in the research could encompass field trials of poplar and other bioenergy and food plants in many different locations and growing conditions to analyze long-term success, Long said.

Opening a new avenue of scientific thinking

“Conserved chimeric genes such as Booster are often disregarded as non-functional, evolutionary artifacts that no longer influence plant processes,” said ORNL’s Biruk Feyissa, who led the gene’s molecular analysis and is first author on the paper. “But here we proved just the opposite.”

“The discovery opens up a new avenue of scientific thinking,” Tuskan added. “We tend to think of photosynthesis as a difficult-to-improve process. But in fact, the molecular machinery surrounding photosynthesis has continued to evolve as plants adapted to their environment. In this case, the exchange of DNA with associated organisms changed a biological process in a fundamental way.”

The discovery was the result of a collaboration between the two DOE centers, where scientists focus on developing improved bioenergy feedstock plants along with efficient methods to process those plants into advanced fuels and products. The research was supported by CBI and CABBI, both sponsored by the United States Department of Energy’s Office of Science Biological and Environmental Research Program.